The present invention relates to artificial illumination, and in particular relates to solid state lamps that produce white light.
Artificial lighting for illumination purposes is incorporated in a wide variety of environments. Major categories include office lighting, home lighting, outdoor lighting for various purposes, signage, indicators, and many others. In the age of modern electricity, the common forms of artificial lighting include (but are not limited to) incandescent, halogen vapor, and fluorescent. All of these have particular advantages and disadvantages, but in certain aspects and they all use relatively large amounts of electricity compared to their light output, and all tend to have obviously finite lifetimes. In particular, the incandescent lamp has been in use in its present form for almost a century, making it one of the longest lived of modern inventions to remain in an early form. In comparison most other early electronic technologies have been replaced by digital electronic counterparts.
The semiconductor era has witnessed the replacement of many types of electrical devices with solid state counterparts. The most obvious is perhaps the replacement of the vacuum tube (almost unknown to present younger generations) with the transistor. Solid state devices, because of their nature and operation, are inherently much more reliable than earlier generations of electronic devices and can have significantly longer lifetimes, typically by a factor of at least 100.
Furthermore, some solid-state devices emit light in operation. The most common is the light emitting diode (LED) in which current is injected across a p-n junction to drive the recombination of electrons and holes with the concurrent production of photons. Depending upon the semiconductor materials from which the diode is formed, and particularly depending upon the bandgap of those materials, different frequencies of light are emitted and are characteristic of the material. For example, gallium arsenide phosphide (GaAsP) represents a well-established material system for light emitting diodes. Depending on the mole fraction of Ga and As, these materials have a bandgap of between about 1.42 and 1.98 electron volts (eV), and will emit light in the infrared, red and orange portions of the electromagnetic spectrum.
In comparison, materials such as silicon carbide (SiC), gallium nitride (GaN), or the related Group III nitride compounds, have wider bandgaps of about 3.0 and about 3.5 eV respectively, and thus generate photons of higher frequency in the blue, violet, and ultraviolet portions of the spectrum.
Because of their reliability, efficiency and relatively low power demands, solid-state light-emitting devices have gained wide acceptance for a number of applications. Because the devices are relatively small, however, and comparatively less bright than more conventional alternatives (incandescent, florescent) their greatest use has been as indicators and other low brightness applications rather than for illumination.
Additionally, some of the LED properties that are favorable in many circumstances (e.g., emission along a narrow band of wavelengths), tend to make LEDs initially less attractive for illumination purposes. For example, LEDs cast only a narrow range of wavelengths. In many circumstances—often because much of color perception depends upon the illumination frequencies—this compares unfavorably with natural light, or even incandescent or florescent light which, because of some of their inherent limitations, actually cast light across a wider range of frequencies than do LEDs.
Two types of technology are used to produce white light from light emitting diodes. In a first, blue light emitting diodes are combined with both red and green light emitting diodes to produce the desired full colors of the visible spectrum, including white light. In the second, higher frequency emitting diodes (e.g., in the ultraviolet, violet and blue range) are used in conjunction with phosphors (typically yellow-emitting) to emit a combination of direct blue light from the diode and yellow light from the phosphor that in combination give white light from the lamp.
The efficiency of a light emitting diode can be characterized in numerous manners, but in general is dependent upon several factors which in practice become cumulative in their positive or negative aspects. For example, for any given amount of current injected into a light emitting diode, some fraction less than 100% of the injected carriers (electrons or holes) will actually recombine. Of those that recombine, another fraction less than 100% will generate photons. Of the photons created, yet another fraction less than 100% will actually be extracted; i.e., leave the diode as visible light. When a phosphor is incorporated, the efficiency is moderated yet again by the conversion efficiency of the phosphor.
Based upon these and other factors, full spectrum light emitting diode devices have the potential to completely replace both incandescent and fluorescent lighting, and inexpensive, readily-manufactured solid-state lamps that emit white light remain a major goal of both researchers and manufacturers.
Accordingly, increasing the desired output of a white light emitting diode for illumination purposes requires increasing one or more of the injection efficiency, the percentage of radiative recombinations, and the amount of photons extracted. Thus, producing brighter total output at relative power levels and across a wider range of the visible spectrum to produce more pleasing effects when in use remains another continuing goal.